Add BrowserClient for spec support

We have `ApiClient` which helps you to test ApiAction specs, but now we need a `BrowserClient` for testing browser based actions when you're not using Flow.

There are a few catches with this that we need to take in to consideration:
* How do we handle sessions?
* How do we handle cookies?
* How do we handle doing a POST where the Content-Type might change from `text/html` to `application/x-www-form-urlencoded ` or even `multipart/form-data`?

Start by locating the existing ApiClient and ApiAction specs to understand the current spec-support behavior. Define how BrowserClient should handle sessions, cookies, and POST content types, then add coverage showing browser-based actions work without Flow.

Written by the indexing model from the issue text.